Method and apparatus for calibrating an IQ modulator

ABSTRACT

The present disclosure relates to a concept for calibrating an IQ modulator. A calibration method comprises setting one or more control values of the IQ modulator corresponding to a desired constellation point of a constellation diagram to generate an IQ modulating signal; mixing the IQ modulating signal with a carrier signal to generate an IQ modulated transmit signal; transmitting the IQ modulated transmit signal towards a predefined object at a predefined location; receiving a reflection of the IQ modulated transmit signal from the predefined object; mixing the received reflection of the IQ modulated transmit signal with the carrier signal to generate a down-converted receive signal; comparing amplitude and/or phase of the down-converted receive signal with the desired constellation point of the constellation diagram; and adjusting the one or more control values of the IQ modulator until a deviation between the amplitude and/or phase of the received down-converted signal and the desired constellation point falls below a predefined threshold.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/196,560, filed Jun. 29, 2016, which claims priority under 35 U.S.C. §119 to German Patent Application No. 102015112392.4, filed on Jul. 29,2015, the contents of which are incorporated by reference herein intheir entirety.

FIELD

Embodiments generally relate to communications systems, and, moreparticularly, to methods and apparatuses for calibrating IQ modulatorsof communications devices, such as radar devices, for example.

BACKGROUND

A modulation technique that lends itself well to digital communicationsis the so-called IQ modulation. Here, “I” denotes the so-called“In-phase” component of a waveform, and “Q” denotes the so-called“Quadrature” component. IQ modulation can be performed using IQmodulators.

An IQ modulator is a critical component in the signal chain for digitaltransmitters. IQ modulators perform the frequency translation that mixesa baseband signal to a desired location in the Radio Frequency (RF)spectrum. An IQ modulator typically comprises a Local Oscillator (LO)input that is split into In-phase (I) and Quadrature (Q) componentswhich are separated by 90°. These two signals may drive separate mixersthat are also driven by I- and Q-baseband signals. The outputs from bothmixers are then summed to provide a modulated carrier either at RF orIntermediate Frequency (IF).

In radar systems, for example, such as Multiple-Input Multiple-Output(MIMO) Frequency-Modulated Continuous-Wave (FMCW) radar systems, an IQmodulator may be used as Single Side-Band (SSB) mixer for an up or downconversion of the LO signal. A complex sinusoidal signal which may begenerated in baseband may be applied as control signal of the IQmodulator in order to shift the LO signal in frequency for an arbitraryvalue. In this way multiple radar transmitters can be activated at thesame time. Such a radar system may be referred to as Frequency-DivisionMultiple-Access (FDMA) FMCW MIMO radar.

The performance of the IQ modulator influences the overall performanceof communications systems such as radar systems. Because of non-idealbehavior of the IQ modulator there is the need for calibration. Modelbased approaches for the calibration of an IQ modulator are well knownbut have their disadvantages.

It is therefore desirable to provide improved techniques for calibratingIQ modulators.

SUMMARY

An embodiment of the present disclosure relates to a method forcalibrating an IQ modulator. The method comprises setting one or morecontrol values of the IQ modulator corresponding to a desiredconstellation point of a constellation diagram to generate an IQmodulating signal. The IQ modulating signal is then mixed with a carriersignal to generate an IQ modulated transmit signal. The IQ modulatedtransmit signal is transmitted towards a predefined object at apredefined location. A reflection of the IQ modulated transmit signal isreceived from the predefined object. The received reflection of the IQmodulated transmit signal is mixed with the carrier signal to generate adown-converted receive signal. An amplitude and/or phase of thedown-converted receive signal is compared with the desired constellationpoint of the constellation diagram. The one or more control values ofthe IQ modulator are adjusted until a deviation between the amplitudeand/or phase of the received down-converted signal and the desiredconstellation point falls below a predefined threshold.

In some embodiments, the comparison with the desired constellation pointand the adjustment of the IQ modulator control value(s) may be performediteratively until the deviation between the amplitude and/or phase ofthe down-converted receive signal and the desired constellation pointfalls below the predefined threshold. Hence, the method may be regardedas in iterative method.

In some embodiments, the acts of the above method may be performediteratively for each constellation point of the constellation diagram inorder to obtain a calibration with respect to all constellation pointsof the constellation diagram.

In some embodiments, the constellation diagram may be a circularconstellation diagram, i.e., the individual constellation points may belocated on a circle in the IQ plane.

In some embodiments, for example related to FMCW radar systems,generating the carrier signal may include varying a carrier frequency ofthe carrier signal according to a predefined carrier frequency ramp.When setting the one or more control values of the IQ modulator, thecontrol values may be kept constant during the duration of one carrierfrequency ramp.

Some embodiments may include performing a time-to-frequency domaintransformation of the down-converted receive signal, for example, usinga Fast Fourier Transformation (FFT).

In some embodiments, one or more IQ modulator control valuescorresponding to the desired constellation point may be stored in acomputer memory when the deviation between the amplitude and/or phase ofthe down-converted receive signal and the desired constellation pointfalls below the predefined threshold.

To reduce computer memory requirements, some embodiments may includeestimating a control value curve using a curve fitting technique. Curvefitting may be performed based on respective adjusted control values ofthe IQ modulator corresponding to respective desired constellationpoints of the constellation diagram. The parameters describing theestimated curve may be stored in a computer memory.

In some embodiments, the IQ modulated transmit signal is transmittedusing a transmitter portion of a transceiver hardware and the reflectionof the IQ modulated transmit signal is received using a receiver portionof the same transceiver hardware. Thereby a transceiver is a devicecomprising both a transmitter and a receiver which are combined andshare common circuitry or a single housing. Hence, no externalmeasurement devices may be required. For example, the transceiverhardware may be embedded in a radar sensor.

In some embodiments, the predefined object for reflecting radiation maybe located in an anechoic chamber.

According to a further aspect, the present disclosure also provides acalibration system. The calibration system comprises an IQ modulator anda controller configured to set one or more control values of the IQmodulator corresponding to a desired constellation point of aconstellation diagram to generate an IQ modulating signal. A first mixerof the calibration system is configured to mix the IQ modulating signalwith an RF carrier signal to generate an IQ modulated transmit signal.Transmitter circuitry of the calibration system is configured totransmit the IQ modulated transmit signal towards a predefined object ata predefined location. Receiver circuitry of the calibration system isconfigured to receive a reflection of the IQ modulated transmit signalfrom the predefined object. A second mixer of the calibration system isconfigured to mix the received reflection of the IQ modulated transmitsignal with the RF carrier signal to generate a down-converted receivesignal. A processor is configured to compare an amplitude and/or phaseof the down-converted receive signal with the desired constellationpoint of the constellation diagram and is further configured to adjustthe one or more control values of the IQ modulator until a deviationbetween the amplitude and/or phase of the received down-converted signaland the desired constellation point falls below a predefined threshold.

In some embodiments, the calibration system may be implemented in aradar transceiver or at least include a radar transceiver. Thetransmitter and receiver circuitry may both be portions of the sameradar transceiver. In other words, transmitter and receiver circuitrymay be collocated in the same housing. Hence, no further externalmeasurement devices may be required.

In some embodiments, the radar transceiver is a FMCW radar transceiver,in particular a multichannel FMCW radar transceiver, and comprises acarrier signal generator which is configured to generate the carriersignal by varying a carrier frequency of the carrier signal according toa predefined carrier frequency ramp over a predefined period of time.

In some embodiments, the processor is configured to perform an FFT ofthe down-converted receive signal and to derive I and Q values of areceived constellation point from the frequency domain signal.

Embodiments of the present disclosure do not require additionalmeasurement equipment besides a transceiver including the IQ modulator,transmitter, and receiver. There is no need for knowledge of amplitudeand phase behavior over the whole IQ-range. The iteration process maydeliver IQ-settings right at the power level of interest.

BRIEF DESCRIPTION OF THE FIGURES

Some embodiments of apparatuses and/or methods will be described in thefollowing by way of example only, and with reference to the accompanyingfigures, in which

FIGS. 1A and 1B show block diagrams of a single channel FMCW radarsystem;

FIGS. 2A and 2B illustrate an example of a multichannel Time DivisionMultiple Access (TDMA) MIMO radar system;

FIGS. 3A and 3B show an example of a single channel FMCW radar system,which can be operated in heterodyne mode;

FIGS. 4A and 4B show an example of a dual channel FDMA FMCW radarsystem;

FIGS. 5A-5D illustrate an IQ modulator, a constellation diagram, andsources of error;

FIG. 6A shows an example of ghost targets;

FIG. 6B illustrates a conventional measurement setup used forcalibration of an IQ modulator;

FIG. 7 shows a flow-chart of a method for calibrating an IQ modulatoraccording to an embodiment;

FIG. 8A illustrates a calibration setup according to an embodiment;

FIG. 8B illustrates a second step in a calibration process according toan embodiment and an iterative process of finding DAC generated controlsignals for a desired constellation point on RX side;

FIG. 9 shows a consecutive process of finding the DAC generated controlsignals for all desired constellation points on RX side;

FIG. 10A shows DAC settings for perfect circle on RX side and ellipseestimated with LS-fitting; and

FIG. 10B illustrates phase differences Δϕ of DAC settings for perfectcircle on RX side and ellipse estimated with LS-fitting.

DETAILED DESCRIPTION

Various example embodiments will now be described more fully withreference to the accompanying drawings in which some example embodimentsare illustrated. In the figures, the thicknesses of lines, layers and/orregions may be exaggerated for clarity.

Accordingly, while further embodiments are capable of variousmodifications and alternative forms, some example embodiments thereofare shown by way of example in the figures and will herein be describedin detail. It should be understood, however, that there is no intent tolimit example embodiments to the particular forms disclosed, but on thecontrary, example embodiments are to cover all modifications,equivalents, and alternatives falling within the scope of thedisclosure. Like numbers refer to like or similar elements throughoutthe description of the figures.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between,” “adjacent” versus “directlyadjacent,” etc.).

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting of furtherexample embodiments. As used herein, the singular forms “a,” “an” and“the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. It will be further understood thatthe terms “comprises,” “comprising,” “includes” and/or “including,” whenused herein, specify the presence of stated features, integers, steps,operations, elements and/or components, but do not preclude the presenceor addition of one or more other features, integers, steps, operations,elements, components and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which example embodiments belong. Itwill be further understood that terms, e.g., those defined in commonlyused dictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art, unlessexpressly defined otherwise herein.

In the following, some basics of FMCW radar systems, including singlechannel as well as multichannel (MIMO) operation, are briefly described.Such FMCW radar systems may be considered as one possible application ofembodiments of the present disclosure. It should be noted that theproposed calibration method is based on the characteristics of the FMCWradar principle using a calibration target and a frequency ramp withhigh bandwidth (e.g. 1 GHz) However, the skilled person having benefitfrom the present disclosure will appreciate that embodiments may also beapplicable to communications systems using IQ modulators other thanradar systems. The concept proposed herein may be beneficial forcalibrating transceivers using IQ modulators in the transmit (TX) path.Such transceivers may be found in FMCW radar systems and various otherwireless communication devices, such as cellular phones, for example.

FIGS. 1A and 1B show a schematic block diagram of a single channel FMCWradar system or transceiver 100.

The FMCW radar transceiver 100 includes a transmitter (TX) portion and areceiver (RX) portion, both using the same continuous wave carrierfrequency f_(LO)(t). The skilled person will appreciate that carrierfrequency f_(LO)(t) is a frequency in the RF range, for example 77 GHz.Further, the carrier frequency f_(LO)(t) is a frequency modulatedcontinuous wave (FMCW) carrier frequency, i.e., the carrier frequencyf_(LO)(t) varies within a certain bandwidth B_(SW). In the illustratedexample, the carrier frequency f_(LO)(t) ramps up and down according toa saw-tooth waveform. Note that in principle also other frequency rampsof the carrier frequency are possible, such as sine waves or triangularwaves.

In the illustrated example, the linearly increasing RF frequency ramp isgenerated by a Voltage Controlled Oscillator (VCO) 108 which iscontrolled and stabilized by a Phase Locked Loop (PLL) synthesizer 104in combination with a loop filter 106 and a reference clock 102. Theoutput signal of the VCO 108, i.e. the FMCW carrier signal havingfrequency f_(LO)(t), is distributed to the input of a Power Amplifier(PA) 110 in the TX path and to an LO port of an RX mixer 118. Theamplified LO signal is transmitted via TX antenna 112, reflected byobject 130, received via RX antenna 114, and amplified by a Low NoiseAmplifier (LNA) 116 in the RX portion of the FMCW radar transceiver 100.The received and amplified signal is down converted to baseband orIntermediate Frequency (IF). In the example of FIGS. 1A and 1B an IQ RXmixer 118 is used. A resulting complex valued baseband or IF-signals_(IF)(t) is band-pass filtered 120 and digitized by Analog-to-DigitalConverters (ADCs) 122. For digital signal processing a Fast Fouriertransform (FFT) may be applied to the sampled baseband or IF data.

To illustrate the basic principle of an FMCW radar the transmitted andreceived RF waveforms of three consecutive frequency ramps are shown inthe lower portion of FIGS. 1A and 1B. The resulting baseband orIF-signal s_(IF)(t) for a static single target scenario is a sinusoidalsignal with a frequency f_(IF) which is proportional to the distancebetween target 130 and radar transceiver 100.

To increase the resolution of radar systems, e.g. to estimate an angularposition of a target, so-called multichannel radar concepts may beemployed. Turning now to FIGS. 2A and 2B, an example of a multichannelTime Division Multiple Access (TDMA) MIMO radar transceiver 200 will beexplained.

The illustrated example radar transceiver 200 comprises two transmitpaths TX₁, TX₂ as well as two receive paths RX₁, RX₂. The skilled personhaving benefit from the present disclosure will appreciate that alsomore TX paths and/or RX paths could be implemented. The MIMO principlerequires the use of orthogonal TX waveforms in order to be able toseparate respective signal reflections at the receiver. This means thatin the RX signal the target responses caused by the differenttransmitters should be clearly assignable to the corresponding TX path.

A common and cost efficient solution is shown in FIGS. 2A and 2B. Thetwo transmit paths TX₁, TX₂ are activated in consecutive time intervals.During a first time interval transmit path TX₁ is activated or enabledwhile transmit path TX₂ is mute. During a second consecutive timeinterval transmit path TX₂ is activated while transmit path TX₁ is mute,and so on. In contrast, the two receive paths RX₁, RX₂ are active orenabled simultaneously to receive slightly phase-shifted signalreflections from target 130 due to slightly different locations of theRX antennas 114-1, 114-2. In the TDMA MIMO radar concept of FIGS. 2A and2B two consecutive ramps are necessary to get all required data (allcombinations of transmitters and receivers) for Digital Beam Forming(DBF). Known methods of DBF may be used to estimate an angular positionof the target 130. For that purpose phase differences caused bydifferent round-trip delay-times (RTDTs) (different paths from allcombinations of transmitters and receivers) are taken into account.Various DBF techniques are well known in the art. Therefore a detaileddescription of DBF is omitted for the sake of brevity.

To avoid the rather time-consuming consecutive switching between thetransmit paths TX₁, TX₂ an IQ modulator may be used as Single-Side-Band(SSB) mixer in the TX path. The IQ modulator concept will be describedwith regard to an example single channel heterodyne FMCW radartransceiver 300 of FIGS. 3A and 3B and an example dual channel FDMA FMCWradar transceiver 400 of FIGS. 4A and 4B.

In FIGS. 3A and 3B, the FMCW carrier signal with varying frequencyf_(LO)(t) is additionally shifted in frequency by an arbitrary frequencyoffset value f_(offset). For that purpose the TX path of FDMA FMCW radartransceiver 300 comprises an IQ modulator 340 which generates an IQmodulating signal s_(M)(t) with a frequency corresponding to thefrequency offset f_(offset). This can be done, for example, by using anIQ modulating signal s_(M)(t) generated from a circular constellationdiagram. Circular constellation diagrams are known as representations ofsignals modulated by digital Phase-Shift Keying (PSK) modulationschemes, for example. In general, a constellation diagram displays thesignal as a two-dimensional scatter diagram in the complex plane atsymbol sampling instants. In a more abstract sense, it represents thepossible symbols that may be selected by a given modulation scheme aspoints in the complex plane. To obtain an IQ modulating signal s_(M)(t)with a frequency f_(offset) subsequent digital I and Q baseband samplesof the IQ modulating signal s_(M)(t) may be chosen to correspond toadjacent constellation points of the circular constellation diagram inclockwise or counterclockwise direction. Adjacent constellation pointsof the circular constellation diagram may be separated in phase by360°/M, wherein M denotes the size of the modulation symbol alphabet.For 8-PSK, for example, M=8.

The digital I and Q baseband samples corresponding to constellationpoints of the constellation diagram may be converted to analog signalsby respective Digital-to-Analog Converters (DACs) 342-I, 342-Q. Theoutput of the DACs may be controlled by respective control settings. Theresulting analog I and Q signals constituting the modulating signals_(M)(t) may be low-pass filtered, respectively, before mixing thefiltered modulating signal s_(M)(t) with the FMCW carrier signalgenerated by VCO 108 to generate an IQ modulated transmit signal. Notethat the IQ modulated transmit signal is a FMCW carrier signal with afrequency ramp and an additional frequency offset f_(offset). The IQmodulated transmit signal is then amplified (PA 110) and transmitted viaTX antenna 112, and reflected by object 130.

As has already been described, the reflected signal is received via RXantenna 114 and amplified by LNA 116. The received and amplified signalis down converted to baseband or IF. In the example of FIG. 3 an IQ RXmixer 118 is used. The resulting complex valued baseband or IF-signals_(IF)(t) is band-pass filtered 120 for extracting the beat frequency ofthe target response and digitized by ADCs 122. Again, an FFT may beapplied to the sampled baseband or IF data. Compared to FIGS. 2A and 2B,however, the target response is now also shifted by the valuef_(offset). The reason is that the received signal is down-converted bythe unmodulated FMCW carrier signal with frequency f_(LO)(t), as can beseen in FIGS. 3A and 3B.

Applying different frequency offsets f_(offset,1), f_(offset,2) todifferent TX paths TX₁, TX₂ may enable orthogonality and hencesimultaneous activation and operation of the different TX paths TX₁,TX₂, hence FDMA. In fact the TX signals of TX paths TX₁, TX₂ are nearlyorthogonal, as they are only orthogonal within an unambiguous range,which is defined as the bandwidth between two consecutive offsetfrequencies. The target responses caused by the different transmittersTX₁, TX₂ are then separated in frequency due to the different frequencyoffsets f_(offset,1), f_(offset,2). As can be seen from the example ofFIGS. 4A and 4B, only one measurement interval corresponding to only onefrequency ramp is necessary to get all information required for DBF.

FIG. 5A shows a block diagram of an example implementation of an IQmodulator 340.

The LO-signal, e.g. the FMCW carrier signal generated by VCO 108, issplit in two signal paths (I-path and Q-path). A relative phase shift of90° is applied between the I- and the Q-paths. These phase shiftedLO-signals are used as input signals of two mixers 346-I, 346-Q. Themixer 346-I in the I-path is used for the up conversion of theI-component Re{s_(M)(t)} and the other 346-Q is used for the upconversion of the Q-component Im{s_(M)(t)} of the modulating signals_(M)(t).

An example of a 4-QAM (QAM: Quadrature Amplitude Modulation)constellation diagram 500 also shown in FIG. 5A. For 4-QAM theconstellation diagram 500 has four constellation points, as can be seenfrom FIG. 5A. Each complex-valued constellation point can be representedby its I- and Q-components. Measured constellation diagrams can be usedto recognize the type of interference and distortion in a signal.

Typically IQ modulators have non ideal behavior. Common sources of errorand their influence on RF signals are illustrated by measuredconstellation diagrams shown in FIGS. 5B, 5C, and 5D. FIG. 5Billustrates the effect of DC Offset in 4-QAM and 16-QAM signals, FIG. 5Cillustrates the effect of quadrature skew in 4-QAM, 16-QAM signals, andFIG. 5D illustrates the effect of IQ gain imbalance in 4-QAM, 16-QAMsignals.

As described above, IQ modulators may be used to generate orthogonal TXwaveforms for multichannel FDMA FMCW radar transceivers, for example.Therefore different frequency offsets f_(offset,channel) may be appliedto the LO signal using the IQ modulator as an SSB mixer. The performanceof the IQ modulators directly influences the achievable radar systemperformance. Unwanted harmonics caused by non-ideal behavior (e.g.,non-linearities) of the IQ-modulators may appear in the calculated IFspectra (range compression) as ghost targets (see FIG. 6A).

Conventional calibration methods are based on model assumptions.Measurements of constellation diagrams, such as shown by FIGS. 5B-5D,are used to calculate predistortion parameters to compensate for DCoffsets, quadrature skew, and IQ gain imbalances. Often separate andexternal measurement equipment has to be used to carry out the requiredmeasurements. Conventional model based calibration approaches may not besufficient to achieve the high suppression of spectral components causedby nonlinearities as they only account for DC offsets, quadrature skew,and IQ gain imbalances.

FIG. 6B shows a conventional radar measurement setup 600 which iscommonly used for calibration of an IQ modulator. The setup 600comprises a single channel FMCW radar transceiver 300 and separateexternal measurement equipment 610. Note that the frequency of the RFsignal is kept constant during this measurement.

The DACs 342-I, 342-Q of FMCW radar transceiver 300 generate controlsignals corresponding to constellations points of a certainconstellation diagram 650. For the control signal generation it isassumed that the IQ modulator 340 and the TX path have ideal behavior.Based on the externally measured receive (RX) constellation diagram 660one or more pre-distortion parameters may be calculated. For thatpurpose a predefined model is typically used which takes the DC offsets,quadrature skew, and IQ gain imbalances into account.

Embodiments proposed in the present disclosure aim at improving suchconventional calibration concepts in order to reduce sources of errorscaused by non-linearities that are not covered by model basedcalibration approaches.

Turning now to FIG. 7, a method 700 for calibrating an IQ modulator 340according to an embodiment will be described in more detail. The skilledperson having benefit from the present disclosure will appreciate thatthe IQ modulator may be an IQ modulator of a radar transceiver or an IQmodulator or another transceiver device.

Method 700 includes the following acts:

-   -   setting 702 one or more control values of the IQ modulator 340        corresponding to a desired constellation point of a        constellation diagram to generate an IQ modulating signal        s_(M)(m), with m denoting the constellation point index,    -   mixing 704 the IQ modulating signal s_(M)(m) with a carrier        signal to generate an IQ modulated transmit signal,    -   transmitting 706 the IQ modulated transmit signal towards a        predefined object or target at a predefined location,    -   receiving 708 a reflection of the IQ modulated transmit signal        from the predefined object or target,    -   mixing 710 the received reflection of the IQ modulated transmit        signal with the carrier signal to generate a down-converted        receive signal (e.g. a target response),    -   comparing 712 an amplitude and/or phase of the down-converted        receive signal (e.g. the target response) with the desired        constellation point of the constellation diagram,    -   adjusting 714 the one or more control values of the IQ modulator        until a deviation between the amplitude and/or phase of the        received down-converted signal (e.g. the target response) and        the desired constellation point falls below a predefined        threshold.

Thereby the acts of method 700 may be performed iteratively for eachdesired constellation point of the constellation diagram until thedeviation between the amplitude and/or phase of the down-convertedreceive signal and the desired constellation point falls below thepredefined threshold. If the constellation diagram comprises more thanone constellation point (which will be the typical case) the respectivesequence of acts 702-714 may be performed iteratively to calibratecontrol values of the IQ modulator for each constellation point ofinterest.

When calibrating FMCW radar systems the carrier signal may be an RFcarrier signal generated by varying a carrier frequency of the carriersignal according to a predefined carrier frequency ramp. In other words,the RF carrier signal may be a frequency ramp. As has been explainedbefore, the carrier frequency ramp may have a sawtooth-like shape, forexample. To generate an additional frequency offset f_(offset,channel)to the LO signal (carrier signal) by mixing it with the IQ modulatingsignal s_(M)(t) the constellation diagram is preferably a circularconstellation diagram.

A calibration system 800 according to an embodiment is shown in FIG. 8A.The calibration system 800 may perform calibration method 700. Thisexample setup includes a FMCW radar transceiver 300 and a predefinedobject/target 130 at a predefined location.

Radar calibration system 800 comprises an IQ modulator 340. A controller840 is configured to set one or more control values, e.g. DAC controlvalues, of the IQ modulator 340 corresponding to a desired constellationpoint 852 of a constellation diagram 850 to generate an IQ modulatingsignal s_(M)(t). A first IQ mixer 346 is configured to mix the I- andQ-components of the IQ modulating signal s_(M)(t) with respective I- andQ-components of a FMCW carrier signal to generate an IQ modulatedtransmit signal. Transmitter circuitry including PA 110 and TX antenna112 is configured to transmit the IQ modulated transmit signal towardsthe predefined object 130 at a predefined location. For predefinedreflection conditions the predefined object 130 may be located in ananechoic chamber, for example. Receiver circuitry of the radartransceiver 300 including RX antenna 114 and LNA 116 is configured toreceive a reflection of the IQ modulated transmit signal from thepredefined object 130. A second IQ mixer 118 is configured to mix thereceived reflection of the IQ modulated transmit signal with the (nonIQ-modulated) FMCW carrier signal to generate a down-converted receivesignal. The down-converted receive signal may be a baseband signal. Aprocessor 860 is configured to compare an amplitude and/or phase of thedown-converted receive signal with the desired constellation point 852of the constellation diagram 850 and is configured to adjust the one ormore control values of the IQ modulator 340 until a deviation betweenthe amplitude and/or phase of the received down-converted signal and thedesired constellation point 852 falls below a predefined threshold. Insome embodiments, the processor 860 and the controller 840 may becoupled via an interface for closing a feedback-loop. That is, theprocessor 860 may instruct the controller 840 to adjust the IQ modulatorcontrol signals (e.g., DAC settings).

In the setup of FIG. 8A, the IQ modulated transmit signal is transmittedusing a transmitter portion of a transceiver hardware, while thereflection of the IQ modulated transmit signal is received using areceiver portion of the same transceiver hardware. Said transceiverhardware is embedded in the same radar sensor 300 in the illustratedembodiment. Hence, no external measurement equipment is needed.

At the starting point of the proposed iterative process 700 the desiredconstellation diagram 850 at the baseband or IF-output is defined. Assome embodiments of the method 700 are intended to be used for FDMA FMCWradar operation, a circular constellation diagram 850 may be chosen (seeFIG. 8A). Here, the example constellation diagram 850 comprises twelveconstellation points. However, any number of constellation points ispossible. The proposed calibration method requires the use of a staticcalibration target 130 located at a fixed range from the radartransceiver 300 to be calibrated. FMCW radar measurements are carriedout which means that during the duration of one RF ramp the controlsignals of the IQ modulator 340 remain constant. A time-to-frequencytransformation, for example an FFT, of the sampled IF-data may beperformed in order to estimate the values of I and Q of the receivedconstellation point. In the spectra of the range compressed data theamplitude and phase information can be found at the peak value of thetarget response which are equivalent to the I and Q values of theconstellation point.

The iterative calibration process starts with the calibration of IQmodulator control signals for a first constellation point m=1, as shownin FIG. 8B. The start value of the control signal(s) (Re{s_(M)(m)};Im{s_(M)(m)}, with m=1 denoting the first constellation point) of the IQmodulator 340 is marked as cross 871. If the deviation between thedesired (e.g. ideal) constellation point and actually received (RX)constellation point 881 is too high, the IQ modulator control signal(s)of the first constellation point is modified and the resulting RXconstellation point is measured again (circles 882, 883) at the receiverside. This procedure may be repeated until the desired constellationpoint 884 is received. The required predistorted IQ modulator controlsignals (e.g., TX DAC settings) 874 corresponding to the desired RXconstellation point 884, hence the predistorted control value(s)(Re{s_(M)(m)}; Im{s_(M)(m)}, with m=1 denoting the first constellationpoint), may be stored in a memory.

This iterative procedure may be repeated for all other constellationpoints m=2 . . . 12 of the example constellation diagram 850, as shownin FIG. 9. A result of this iterative calibration procedure is shown inFIG. 10A. The required predistorted IQ modulator control signals (e.g.,TX DAC settings) for the desired RX constellation points are marked withcircles in the upper predistorted constellation diagrams, while thecorresponding RX constellation points are marked with circles in thelower predistorted constellation diagrams of FIG. 9. It can be seen inthe illustrated example that the predistorted IQ modulator controlsignals, i.e., the predistorted TX constellation diagram 900, have anellipsoid form, while the RX constellation points are located on adesired circle 910.

In an optional further act of method 700, a curve fitting based onrespective adjusted or predistorted control values of the IQ modulatorcorresponding to respective desired RX constellation points of theconstellation diagram may be performed. The parameters describing theestimated predistortion curve may be stored in a computer memory, forexample. In some embodiments related to circular constellation diagrams,a Least-Square (LS) ellipse fit may be performed, for example. TheLS-estimated positions of the predistorted IQ modulator control signalsthat are required for a perfect RX constellation diagram are marked withcrosses in FIG. 10A. Example ellipse parameters of the LS-fitting can befound in Table 1.

TABLE 1 Ellipse parameters. a semi-major axis b semi-minor axis Z₁center Z_(Q) center φ twisting angle ϕ angle relative to semi-major axis

As can be seen from FIG. 10B, the positions of the predistorted IQmodulator control signals or DAC settings in the constellation diagramrequired for a perfect RX circle (circles) differ from the estimatedLS-positions on the LS-estimated ellipse (crosses). This is caused bynonlinearities of the IQ modulator. In an optional additional step ofthe calibration method, the phase differences Δϕ_(i) between the foundpredistorted IQ modulator control values (for example, DAC settings) fora perfect RX constellation circle and the estimated LS-positions may becalculated for each constellation point i of the IQ modulator controlvalues (see FIG. 10B). After that, a function Δϕ=f(ϕ) with inputparameter ϕ for the phase deviation Asp may be calculated and stored bymethods of regression analysis, for example. In this way, the optimumpredistorted IQ modulator control values (for example, DAC settings) maybe set during normal operation based on the stored ellipse parametersand based on the respective predistorted IQ modulator control value'sangle ϕ relative to semi-major axis (to obtain Δϕ=f(ϕ)).

To summarize, some embodiments propose to characterize/calibrate atransmitter by performing a real radar measurement with the completeradar system. The radar system may be put into a well-known environment,e.g. an anechoic chamber with one static radar target with known radarcross section (for example, a corner reflector). The measurement resultsmay then be used to characterize the transmitter and derive the optimumcompensation parameters.

The measurement may be performed by transmitting an FMCW ramp coveringthe desired operating bandwidth B_(SW), and receiving the reflectedsignal by the system under test. The complete radar system is involvedso that any imperfections which might be present in the transmitter,receiver, antennas, signal processing chain, etc. can be covered by theproposed calibration concept. The measurement is also covering thecomplete frequency band B_(SW) of interest, intrinsically weighting andaveraging over frequency-dependent effects in exactly the same way asthe operating radar does. Furthermore, a real radar measurement isperformed, requiring no other external RF equipment and utilizing theRF, analog and digital signal processing of the system under test. Inthis way, the desired data may be collected in the fastest possible way(for example, ‘real-time’).

The received signal will contain the echo from the reflector at aprecisely known frequency. The amplitude and phase of this echo containsthe desired data for instantaneous (or later) compensation, and may becollected by the test setup (for example, a computer) which controls theradar system for this characterization. The amplitudes and phases of thereceived echo, which correspond to the transmitter settings, are used toobtain the required pre-distortion parameters.

The proposed real-time approach tries try to find the correct I/Qmodulator settings to obtain the desired RF constellation points by aniterative process. The settings of the I/Q modulator may be varied (byan iterative method minimizing the remaining error) until the desiredoptimum constellation is received.

To get the full characteristics of a transmitters amplitude and phasemodulator (or I/Q modulator), a reasonable number of modulation settingsmay be applied to the transmitter, and one radar measurement may beperformed for each setting. In one embodiment, the results (of bothmethods) are the parameters of an elliptic pre-distortion for I and Q.In combination, a phase pre-distortion is applied.

Care has to be taken that the distance to the corner reflector does notchange, which is usually guaranteed by a rigid mechanical connectionbetween radar system and corner reflector. Thus, any deviation inamplitude and phase between transmit setting and received signaloriginates from the system itself and can be compensated since it is nowprecisely known.

Embodiments of the present disclosure propose usage of the alreadyexisting hardware in a transceiver, which may be a radar transceiver,for example. The desired mode of transceiver operation (e.g. FDMA FMCWradar) may be used. In the proposed approach, an iteration process maybe used to find the perfect settings for the control signals s_(M)(t) ofthe IQ modulator in the complex IQ plane to achieve the best receivedsignal. An identical signal (e.g. power level of the signal) may thenalso be used in normal radar operation. The iterative process has veryquick convergence and has very low calculation complexity.

The description and drawings merely illustrate the principles of thedisclosure. It will thus be appreciated that those skilled in the artwill be able to devise various arrangements that, although notexplicitly described or shown herein, embody the principles of thedisclosure and are included within its spirit and scope. Furthermore,all examples recited herein are principally intended expressly to beonly for pedagogical purposes to aid the reader in understanding theprinciples of the disclosure and the concepts contributed by theinventor(s) to furthering the art, and are to be construed as beingwithout limitation to such specifically recited examples and conditions.Moreover, all statements herein reciting principles, aspects, andembodiments of the disclosure, as well as specific examples thereof, areintended to encompass equivalents thereof.

Furthermore, the following claims are hereby incorporated into thedetailed description, where each claim may stand on its own as aseparate example embodiment. While each claim may stand on its own as aseparate example embodiment, it is to be noted that—although a dependentclaim may refer in the claims to a specific combination with one or moreother claims—other example embodiments may also include a combination ofthe dependent claim with the subject matter of each other dependent orindependent claim. Such combinations are proposed herein unless it isstated that a specific combination is not intended. Furthermore, it isintended to include also features of a claim to any other independentclaim even if this claim is not directly made dependent to theindependent claim.

It is further to be noted that methods disclosed in the specification orin the claims may be implemented by a device having means for performingeach of the respective acts of these methods.

Further, it is to be understood that the disclosure of multiple acts orfunctions disclosed in the specification or claims may not be construedas to be within the specific order.

Therefore, the disclosure of multiple acts or functions will not limitthese to a particular order unless such acts or functions are notinterchangeable for technical reasons. Furthermore, in some embodimentsa single act may include or may be broken into multiple sub acts. Suchsub acts may be included and part of the disclosure of this single actunless explicitly excluded.

What is claimed is:
 1. A method for calibrating an in-phase quadrature(IQ) modulator, the method comprising: setting one or more controlvalues, of the IQ modulator, corresponding to a constellation point of aconstellation diagram to generate an IQ modulating signal; mixing the IQmodulating signal and a carrier signal; generating based on mixing theIQ modulating signal and the carrier signal, a down-converted signal;comparing an amplitude and/or phase of the down-converted signal and theconstellation point; and adjusting the one or more control values of theIQ modulator until a deviation between the amplitude and/or phase of thedown-converted signal and the constellation point falls below apredefined threshold.
 2. The method of claim 1, where the carrier signalis a radio frequency (RF) signal generated by varying a carrierfrequency of the carrier signal according to a carrier frequency ramp.3. The method of claim 1, where mixing the IQ modulating signal with thecarrier signal comprises: mixing an I-component and a Q-component, ofthe IQ modulating signal, with a respective I-component and Q-componentof the carrier signal.
 4. The method of claim 1, where mixing the IQmodulating signal with the carrier signal comprises: mixing a reflectionof the IQ modulating signal with a non-IQ-modulated carrier signal. 5.The method of claim 1, where setting the one or more control values, ofthe IQ modulator, corresponding to the constellation point of theconstellation diagram comprises: iteratively setting one or more controlvalues, of the IQ modulator, corresponding to each constellation pointof the constellation diagram; and where adjusting the one or morecontrol values of the IQ modulator comprises: calibrating the one ormore control values of the IQ modulator for each constellation point ofthe constellation diagram.
 6. The method of claim 1, where adjusting theone or more control values of the IQ modulator comprises: calibrating,based on the deviation not satisfying the predefined threshold, the oneor more control values of the IQ modulator for another constellationpoint of the constellation diagram.
 7. The method of claim 1, where themethod is performed by a device that comprises the IQ modulator.
 8. Asystem, comprising: one or more devices to: set one or more controlvalues, of an in-phase quadrature (IQ) modulator, corresponding to aconstellation point of a constellation diagram to generate an IQmodulating signal; mix the IQ modulating signal and a carrier signal;generate a down-converted signal based on mixing the IQ modulatingsignal and the carrier signal; compare an amplitude and/or phase of thedown-converted signal and the constellation point of the constellationdiagram; and adjust the one or more control values of the IQ modulatoruntil a deviation between the amplitude and/or phase of thedown-converted signal and the constellation point falls below apredefined threshold.
 9. The system of claim 8, where the carrier signalis a radio frequency (RF) signal generated by varying a carrierfrequency of the carrier signal according to a carrier frequency ramp.10. The system of claim 8, where the one or more devices, when mixingthe IQ modulating signal with the carrier signal, are to: mix anI-component and a Q-component, of the IQ modulating signal, with arespective I-component and Q-component of the carrier signal.
 11. Thesystem of claim 8, where the one or more devices, when mixing the IQmodulating signal with the carrier signal, are to: mix a reflection ofthe IQ modulating signal with a non-IQ-modulated carrier signal.
 12. Thesystem of claim 8, where the one or more devices, when setting the oneor more control values, of the IQ modulator, corresponding to theconstellation point of the constellation diagram, are to: iterativelyset one or more control values, of the IQ modulator, corresponding toeach constellation point of the constellation diagram; and where the oneor more devices, when adjusting the one or more control values of the IQmodulator, are to: calibrate the one or more control values of the IQmodulator for each constellation point of the constellation diagram. 13.The system of claim 8, where the one or more devices, when adjusting theone or more control values of the IQ modulator, are to: calibrate, basedon the deviation not satisfying the predefined threshold, the one ormore control values of the IQ modulator for another constellation pointof the constellation diagram.
 14. The system of claim 8, where thesystem comprises the IQ modulator.
 15. A device, comprising: acontroller configured to set one or more control values, of an in-phasequadrature (IQ) modulator, corresponding to a constellation point of aconstellation diagram to generate an IQ modulating signal; a first mixerconfigured to mix the IQ modulating signal and a carrier signal; asecond mixer configured generate a down-converted signal based on mixingthe IQ modulating signal and the carrier signal; and a processorconfigured to: compare an amplitude and/or phase of the down-convertedsignal and the constellation point of the constellation diagram, andadjust the one or more control values of the IQ modulator until adeviation between the amplitude and/or phase of the down-convertedreceive signal and the constellation point falls below a predefinedthreshold.
 16. The device of claim 15, where the carrier signal is aradio frequency (RF) signal generated by varying a carrier frequency ofthe carrier signal according to a carrier frequency ramp.
 17. The deviceof claim 15, where the first mixer, when mixing the IQ modulating signalwith the carrier signal, is to: mix an I-component and a Q-component, ofthe IQ modulating signal, with a respective I-component and Q-componentof the carrier signal.
 18. The device of claim 15, where the secondmixer, when generating the down-converted signal, is to: generate thedown-converted signal by mixing a reflection of the IQ modulating signalwith a non-IQ-modulated carrier signal.
 19. The device of claim 15,where the controller, when setting the one or more control values, ofthe IQ modulator, corresponding to the constellation point of theconstellation diagram, is to: iteratively set one or more controlvalues, of the IQ modulator, corresponding to each constellation pointof the constellation diagram; and where the processor, when adjustingthe one or more control values of the IQ modulator, is to: calibrate theone or more control values of the IQ modulator for each constellationpoint of the constellation diagram.
 20. The device of claim 15, wherethe processor, when adjusting the one or more control values of the IQmodulator, is to: calibrate, based on the deviation not satisfying thepredefined threshold, the one or more control values of the IQ modulatorfor another constellation point of the constellation diagram.